When a lane change from a lane on which a vehicle is traveling, hereinafter referred to as own lane, to an adjacent lane is about to be made, existing techniques have controlled the position of the vehicle according to a spring-mass-damper model as a mathematical model that schematizes the motion of a mechanical system by using respective elements: a spring, a mass, and a damper so that the vehicle is brought to an optimum position between vehicles on the adjacent lane, and has allowed the lane change to be made at a time when an inter-vehicle distance on the adjacent lane has become equal to or larger than a threshold value while the vehicle was in the present position (see Japanese Patent No. 4366419).